91
Figure 3-25. Central routing model for increased capacity
There are many different ways to connect such a Core. Figure 3-25
shows a Core with four routers that are interconnected with a full mesh. I have already indicated that a full mesh does not scale well, so if the
network will need further expansion, full mesh would not be a good option. Figure 3-26
shows a similar network but with six central routers connected to one another by a pair of central switches.
Note that there need be no VLANs defined on these two switches. Both switches S1 and S2 have connections to all six routers. A natural way to define the IP segments on these switches is to have one
switch carry one subnet and the other carry a different subnet. Then if either switch fails, the dynamic routing protocol takes care of moving all traffic to the second switch.
In this sort of configuration, it is generally useful to make the routers act in tandem. Assuming that Distribution Areas consist of two Distribution switches and several Access switches, you would connect
both switches to both routers in this pair, and you can connect several Distribution Areas to each pair of routers. The actual numbers depend on the capacity of the routers. All connections will be fully redundant.
Then only the Distribution switches that are part of this group of Distribution Areas will connect to this pair of routers. The next group of Distribution Areas will connect to the next pair of Core Routers.
3.7.2 Routers in the Distribution Level
There are two ways to bring the routers into the Distribution Level. One is to simply extend the concept shown in
Figure 3-26 and arbitrarily proclaim that the two central switches S1 and S2 are now the Core
and the routers are all in the Distribution Level. The distinction between Core and Distribution Levels is somewhat vague and depends partially on where you draw the lines. One problem with this way of
drawing the lines is that these routers interconnect different Distribution Areas, so it is a little tenuous to claim that they are part of the Distribution Level.
92
Figure 3-26. Central routing and switching
The second way of bringing routers into the Distribution Level is to have one or preferably two, for redundancy router for each Distribution Area. This option is shown in
Figure 3-27 .
Figure 3-27. Distributed routing and central switching
One advantage to this approach is that it provides a very natural application of Layer 3 switching. Each Distribution switch could contain a Layer 3 switching module. This way, you can provide efficient VLAN-
to-VLAN communication within each Distribution Area. You would then construct two additional VLANs on each Distribution switch that would connect to the two central switches.
In this sort of model, where routing functions are downloaded to the Distribution Level, another sort of efficiency can be used. Since how you decide which VLANs comprise a VLAN Distribution Area is
somewhat arbitrary, you can deliberately choose your areas to limit traffic through the Core. This may not always be practical, particularly if the Distribution Areas are selected for geographical reasons. If it can be
done, though, it may radically improve the network performance through the Core.
3.7.3 Routers in Both the Core and Distribution Levels
Its pretty clear that the network shown in Figure 3-27
has good scaling properties, but there are limits to even this model. In
Chapter 6 , I will discuss the IP dynamic routing protocol called OSPF. This protocol
allows IP routers to keep one another informed about how best to reach the networks they are responsible for. There are other dynamic routing protocols but OSPF is an open standard and an industry norm. The
comments that follow turn out to be applicable to most of the alternatives as well.
93
In Figure 3-27
, all of the routers talk directly to one another through the Core switches. In any dynamic routing protocol, every router must know about all of its neighboring routers. It maintains a large table of
these neighbor relationships and has to keep it continuously up to date. The more neighbors it has, the harder this job becomes, with similar scaling properties to a fully meshed network. The usual rule is that
you never want more than 50 routers in one OSPF area. There are exceptions to this rule, as I will discuss in the section on OSPF, but it is never wise to push it too far.
If you want no more than 50 routers in your Core, then you can have no more than 25 VLAN Distribution Areas, since there are two routers in each area. With a capacity of over a thousand users in each
Distribution Area, this is a limit that only large organizations will hit. However, it turns out that it isnt terribly difficult to overcome.
All you need to do is create a hybrid of the two solutions, with routers in the Core and Distribution Layers. Each Core router will handle several Distribution routers to allow excellent scaling properties.
Figure 3-28 shows an example of how this hybrid might work. In this figure, the two Core routers that serve the
Distribution Areas shown are the OSPF Area Border Routers ABR for these Distribution Areas.
Figure 3-28. Distributed routing coupled with central routing
There are two other key advantages to this sort of design. First, it makes it extremely easy to spread the Distribution Areas geographically. In fact, you could even make your Core spread physically throughout a
campus area, or even across several cities. However, doing so is generally not a good plan. The Core in this case represents our OSPF area 0 a concept that I will explain in
Chapter 6 . There can be performance
and reliability problems in a network that has its area 0 dispersed over wide area links. These problems can be overcome with careful tuning of OSPF parameters, but it leads to a network Core that has to be
monitored very closely. A broken link in the Core could have disastrous consequences.
It is actually simpler to have the Core in a single geographical location and to bring the links to the various Distribution Areas via WAN links.
That point leads to the second advantage. It is very easy to integrate a large WAN into this sort of design, as I will show in the next section.
94
3.7.4 Connecting Remote Sites
